Various biosensors have conventionally been proposed to quantify a specific component of a sample such as blood, and the following sensor is known as one example (Japanese Laid-Open Patent Publication No. Sho 63-317096).
This biosensor is produced by forming an electrode system comprising a working electrode, a counter electrode and a reference electrode on an insulating base plate by a method such as screen printing and forming a reaction layer comprising an oxidoreductase and an electron mediator on the electrode system.
When a sample containing a substrate is dropped on the reaction layer of the biosensor thus produced, dissolution of the reaction layer takes place to cause a reaction between the oxidoreductase and the substrate, which is accompanied by reduction of the electron mediator. After the lapse of a certain period of time, the reduced electron mediator is electrochemically oxidized by the electrode system, and based on the oxidation current value obtained, the substrate in the sample solution is quantified.
In the biosensor having the above-described constitution, the oxidoreductase and the electron mediator are usually mixed with each other on the electrode system. This is aimed at saving the trouble of mixing or stirring the sample and causing the reaction between the oxidoreductase and the electron mediator to proceed effectively such that quick quantification of the substrate becomes possible.
However, the biosensor in which the oxidoreductase and the electron mediator are mixed or in contact with each other has such a problem that the response value to a sample solution having a substrate concentration of 0 mg/dl (hereinafter referred to as blank value) is not 0 but higher than 0. Further, another problem is that in comparison with the response value immediately after the production of the biosensor (hereinafter referred to as initial stage), the blank value and the response value to a sample solution having the same substrate concentration increase as the storage period becomes longer. The response value is changed by storage becomes a problem in that the reliability is low.
In view of the above problems, an object of the present invention is to provide a biosensor of which blank value is low, of which response value, particularly blank value, is hardly changed by storage, and of which structure is simple.
Another object of the present invention is to provide a biosensor suited for quantification of cholesterol in particular.